Fluororesins such as polytetrafluoroethylene (hereinafter referred to as “PTFE”) are excellent in properties such as heat resistance, light resistance, chemical resistance, electrical insulation, and slidability. Fillers are often added to fluororesins to improve their properties such as wear resistance, electrical conductivity, and thermal conductivity.
Generally, elongated fluororesin sheets are mass-produced by forming a mixture containing a fluororesin and a forming aid into strip sheets by extrusion molding or the like and then rolling the sheets in their longitudinal direction to a desired thickness. Filler-containing fluororesin sheets also can be obtained by forming a mixture containing a fluororesin, a filler, and a forming aid into sheets and then rolling the sheets. However, the resulting fluororesin sheets may have lower strength due to the filler, which is a solid additive.
Patent Literature 1 discloses a PTFE sheet production method suitable for improving the strength of the sheet. In the production method disclosed in Patent Literature 1, a PTFE sheet is produced by forming a mixture containing a fluororesin, a pore-forming material, and a forming aid into sheets, stacking the sheets to form a stack, and then applying pressure to the stack in its thickness direction to compress it. The pore-forming material is added to form pores but is removed from the resulting PTFE sheet. In Examples of Patent Literature 1, the stack is compressed in a press.